diff -Nru iptables-persistent-1.0.4+nmu2ubuntu1/debian/changelog iptables-persistent-1.0.4+nmu2ubuntu2/debian/changelog
--- iptables-persistent-1.0.4+nmu2ubuntu1/debian/changelog	2019-03-21 17:21:31.000000000 -0300
+++ iptables-persistent-1.0.4+nmu2ubuntu2/debian/changelog	2021-12-10 10:51:47.000000000 -0300
@@ -1,3 +1,12 @@
+iptables-persistent (1.0.4+nmu2ubuntu2) bionic; urgency=medium
+
+  * Add configuration options IP[6]TABLES_RESTORE_NOFLUSH so that
+    existing rules are not flushed on start/load (LP: #1949643)
+    - debian/netfilter-persistent.default: Introduce options
+    - plugins/{15-ip4tables,25-ip6tables}: Check/apply options
+
+ -- Jorge Merlino <jorge.merlino@canonical.com>  Fri, 10 Dec 2021 10:51:47 -0300
+
 iptables-persistent (1.0.4+nmu2ubuntu1) bionic; urgency=medium
 
   * plugins/{15-ip4tables,25-ip6tables}: Adjust plugins code to not hard-fail
diff -Nru iptables-persistent-1.0.4+nmu2ubuntu1/debian/netfilter-persistent.default iptables-persistent-1.0.4+nmu2ubuntu2/debian/netfilter-persistent.default
--- iptables-persistent-1.0.4+nmu2ubuntu1/debian/netfilter-persistent.default	2017-03-17 01:50:20.000000000 -0300
+++ iptables-persistent-1.0.4+nmu2ubuntu2/debian/netfilter-persistent.default	2021-12-10 10:51:47.000000000 -0300
@@ -2,3 +2,8 @@
 # Plugins may extend this file or have their own
 
 FLUSH_ON_STOP=0
+
+# Set to yes for not flushing existing ip[6]tables rules when netfilter-persistent
+# is called with the start parameter
+# IPTABLES_RESTORE_NOFLUSH=yes
+# IP6TABLES_RESTORE_NOFLUSH=yes
diff -Nru iptables-persistent-1.0.4+nmu2ubuntu1/plugins/15-ip4tables iptables-persistent-1.0.4+nmu2ubuntu2/plugins/15-ip4tables
--- iptables-persistent-1.0.4+nmu2ubuntu1/plugins/15-ip4tables	2019-03-21 17:21:31.000000000 -0300
+++ iptables-persistent-1.0.4+nmu2ubuntu2/plugins/15-ip4tables	2021-12-10 10:51:47.000000000 -0300
@@ -14,13 +14,24 @@
 
 rc=0
 
+# Source configuration
+if [ -f "/etc/default/netfilter-persistent" ]; then
+	. /etc/default/netfilter-persistent
+fi
+
 load_rules()
 {
+	if [ "${IPTABLES_RESTORE_NOFLUSH}x" = "yesx" ]; then
+		NOFLUSH='--noflush'
+	else
+		NOFLUSH=''
+	fi
+
 	#load IPv4 rules
 	if [ ! -f /etc/iptables/rules.v4 ]; then
 		echo "Warning: skipping IPv4 (no rules to load)"
 	else
-		/sbin/iptables-restore < /etc/iptables/rules.v4 2> /dev/null
+		/sbin/iptables-restore $NOFLUSH < /etc/iptables/rules.v4 2> /dev/null
 		if [ $? -ne 0 ]; then
 			rc=1
 		fi
diff -Nru iptables-persistent-1.0.4+nmu2ubuntu1/plugins/25-ip6tables iptables-persistent-1.0.4+nmu2ubuntu2/plugins/25-ip6tables
--- iptables-persistent-1.0.4+nmu2ubuntu1/plugins/25-ip6tables	2019-03-21 17:21:31.000000000 -0300
+++ iptables-persistent-1.0.4+nmu2ubuntu2/plugins/25-ip6tables	2021-12-10 10:51:47.000000000 -0300
@@ -14,13 +14,24 @@
 
 rc=0
 
+# Source configuration
+if [ -f "/etc/default/netfilter-persistent" ]; then
+	. /etc/default/netfilter-persistent
+fi
+
 load_rules()
 {
+	if [ "${IP6TABLES_RESTORE_NOFLUSH}x" = "yesx" ]; then
+		NOFLUSH='--noflush'
+	else
+		NOFLUSH=''
+	fi
+
 	#load IPv6 rules
 	if [ ! -f /etc/iptables/rules.v6 ]; then
 		echo "Warning: skipping IPv6 (no rules to load)"
 	else
-		/sbin/ip6tables-restore < /etc/iptables/rules.v6 2> /dev/null
+		/sbin/ip6tables-restore $NOFLUSH < /etc/iptables/rules.v6 2> /dev/null
 		if [ $? -ne 0 ]; then
 			rc=1
 		fi